The present invention relates generally to proportional counters, such as gamma counters and liquid scintillation counters used in the measurement of radiation. More particularly, the invention relates to a novel technique for obtaining a pulse-height or energy spectrum pertaining to the radiation from a sample of material to be analyzed or tested by the instrument.
By way of example of the type of counting instrument to which the present invention may be applied, a gamma counter is described more fully herein. Such a counter includes a radiation detector in the form of a sodium iodide crystal activated with thallium. Gamma rays emitted from a radioactive sample excite some of the electrons in the sodium iodide, and the excited electrons react with the thallium to produce light scintillations. These scintillations are then detected by a multiplier phototube and converted into corresponding electrical pulses. The resultant output pulses from the multiplier phototube should be directly proportional, in amplitude, to the energies of corresponding gamma rays from which the pulses were derived. Gamma counters usually include some means for sorting or filtering the output pulses from the phototube, so that an energy or pulse-height spectrum can be obtained.
By way of background, it should be noted that the energy spectrum that can be obtained by use of a gamma counter does not accurately reflect the energy spectrum of the incident radiation. Gamma rays are essentially monoenergetic, i.e., if a radioactive substance has the characteristic that it emits gamma rays at a particular energy level, every gamma ray from the substance will be emitted at exactly the same energy level. If an energy spectrum relating to the gamma radiation were to be plotted, with a count of detected gamma rays plotted along the vertical axis and the gamma ray energy plotted along the horizontal axis, the resulting spectrum would be a vertical line located at the energy level corresponding to the gamma radiation from the radioactive substance, or would be a number of such vertical lines, if the substance emits gamma radiation at a number of different energy levels. In practice, however, such a spectrum can never be obtained from a gamma counter. The sodium iodide scintillator does not always generate exactly the same number of excited electrons from each incident gamma ray, and the multiplier phototube does not always produce exactly the same amplification each time a scintillation is detected by its photocathode. Consequently, the energy or pulse-height spectrum relating to output from the multiplier phototube of a gamma counter will consist of a bell-shaped gaussian distribution, rather than a vertical line in the spectrum corresponding to the energy level of the incident gamma rays. This distribution is usually referred to as a photopeak in the pulse-height spectrum.
In the measurement of radiation other than gamma radiation, the detected radioactive decay events are generally not monoenergetic, and the actual energy spectrum of the incident radiation will be a true energy distribution. Quantitative knowledge relating to these distributions may be extremely important for some types of tests performed with a proportional counting instrument. In liquid scintillation counters, the true energy of a radioactive decay event may be masked by processes taking place in the liquid in which the radioactive sample is contained, resulting in a shifting of the pulse-height spectrum along the energy axis, and further increasing the need for obtaining an accurate spectrum.
Most proportional counters include one or more pulse-height analyzers connected to receive output pulses from the multiplier phototube. Each pulse-height analyzer has upper and lower discriminator limits or settings which can be adjusted to define a desired "window" in the pulse-height spectrum. The pulse-height analyzer acts essentially as a filter, rejecting pulses which fall outside the selected discriminator settings, and passing pulses which fall within the window to a scaler or counting device. For a given test or experiment using such an instrument, the discriminator settings are first adjusted to define a pulse-height window which covers a desired field of interest in the spectrum. It may be, for example, that only one particular photopeak is of interest, or that the radiation from one particular isotope is to be isolated from the radiation produced by other isotopes present in this sample. In any event, the discriminator limits in a pulse-height analyzer can be adjusted properly only if the operator of the instrument has prior knowledge of the entire energy spectrum pertaining to radiation from the sample.
Knowledge of the complete spectrum characteristics would also be important in the analysis of an unknown sample of radioactive material. The spectrum would then be used for purposes of identification of various isotopes in the sample.
Prior to this invention, there were two basic techniques for obtaining a pulse-height spectrum. First, it could be obtained manually in a single-channel instrument, by setting the upper and lower discriminator limits to cover a narrow window at one end of the spectrum, taking a count of pulses in that counting window, then readjusting the discriminator limits to move the window to successive incremental positions across the entire spectrum. This technique is not only very time consuming, but it requires the continued presence of an operator. A pulse-height spectrum can also be obtained by use of a multi-channel analyzer having a large number of channels, perhaps many hundreds, which can be separately adjusted to provide simultaneous counting in a plurality of narrow and contiguous counting windows. Such instruments are far to expensive for routine installation in medical facilities, for example, and are also too costly for many research applications. Consequently, there is a real need for some alternative technique for obtaining a pulse-height spectrum using a proportional counting instrument. The present invention fulfills this need.